The subject invention is directed to sheet products which are useful as separators in batteries, in particular, separators in lithium batteries to prevent the occurrence of overheating and thermal-runaway.
Storage batteries have at least one pair of electrodes of opposite polarity and, generally, have a series of adjacent electrodes of alternating polarity. The current flow between these electrodes is maintained by an electrolyte which can be acid, alkaline or substantially neutral depending on the nature of the battery system. Separators are located in the batteries between adjacent electrodes of opposite polarity to prevent direct contact between the oppositely charged electrode plates while freely permitting electrolytic conduction. The separator is normally in the form of a thin sheet or film or in certain designs can be in the form of an envelope surrounding each electrode plate of one polarity. It is generally agreed that separators should be (a) thin and light weight to aid in providing a battery of high energy density, (b) resistant to degradation and instability with respect to the battery components with which it is in contact, (c) capable of exhibiting a high degree of electrolytic conductivity (low electrolytic resistance) and (d), in battery systems where appropriate, capable of inhibiting formation and growth of dendrites.
Lithium batteries have distinct advantages over other storage batteries. These batteries are capable of providing much higher power storage densities than other types of batteries, excellent shelf life, high energy density (energy content per unit weight) due to the low atomic weight of lithium metal, and high potential for forming a battery in conjunction with positive electrodesfar removed from the lithium electrode in electromotive series. The battery can be formed in any conventional physical design:-cylindrical, rectangular or disc-shaped "button" cells, and are normally of a closed cell configuration. Such batteries are generally composed of a negative lithium electrode, a positive electrode and a non-aqueous electrolyte. The negative electrode is conventionally lithium metal or its alloy on a support, such as a nickel coated screen. Positive electrodes of various types have been suggested, including metal oxides, such as manganese dioxide or transition metal sulfides such as sulfides of cobalt, nickel, copper, titanium vanadium, chromium, cerium and iron. These may be used singly or as mixtures and may further contain other metal ions, such as alkali metal ions. The positive electrode may further contain carbon and a current collector. The electrolyte is formed of a non aqueous solvent containing a lithium salt. For example, solvents may be acetonitrile, tetrahydrofuran, propylene carbonate, and various sulfones. The lithium salts may be lithium perchlorate, iodide or hexafluroarsenate and the like. An additional, normally passive, component of the battery is a separator membrane located between plates of opposite polarity to prevent contact between such plates while permitting electrolytic conduction.
Separators conventionally used in present battery systems are formed of polymeric films which when placed in an electrolyte or electrolyte system, are capable of exhibiting a high degree of conductivity while being stable to the environment presented by the battery. The films may be macroporous or microporous to thus permit transportation of electrolyte. Examples of such separators include polypropylene sheet which has been stretched and annealed to provide microporosity in the sheet. Such sheets, as disclosed in U.S. Pat. Nos. 3,426,754; 3,558,764; 3,679,538; 3,801,404 and 4,994,335, are normally highly oriented and shrink when subject to heat. Other examples are filled polymeric sheets such as those disclosed in U.S. Pat. Nos. 3,351,495 and 4,287,276 in which the electrolyte is capable of passing through the separator through microporous channels and by the wicking of the filler.
Due to the reactivity of lithium, a major problem encountered with these batteries involves overheating of the cell due to improper use of the cell, e.g. accidentally reversing polarity of battery during charging; contact between electrodes of opposite polarity such as by dendrite formation or shrinkage of separator; formation of high surface spongy lithium which exothermally reacts with the solvent-electrolyte; as well as other known conditions. Such overheating tends to cause thermal runaway and potentially explosive effects as the system continues to act in its defective mode. This must be controlled to provide a battery having commercial acceptability. One method of controlling or preventing thermal runaway is to have the battery casing connected to a sensor which shuts down the electrical system of the apparatus when overheating is detected. This method requires additional electrical circuitry and associated devices and is unable to make early detection of internal cell overheating and preventive effort.
Conventional polymeric films employed as separators lithium batteries are generally not capable of preventing any uncontrolled overheating. Some separator films are inert to heat and, therefore, do not trigger any preventative mechanism. Other presently used separators, such as microporous polyolefins, when subjected to elevated temperatures exhibit dimensional instability and/or degradation permitting contact between large sections of electrodes of opposite polarity which only accelerates the thermal-runaway of a battery.
Recently U.S. Pat. No. 4,650,730 disclosed a sheet product which is useful as a battery separator and has the capability of drastically reducing its porosity at a predetermined elevated temperature. Thus, the separator is capable, upon detection of internal overheating of shutting-down the battery's system by becoming a barrier to the passage of ions between electrodes of opposite polarity. This separator is composed of a sheet product having at least two microporous plies with one ply being a substantially unfilled microporous sheet capable of transforming to a non-porous sheet at a predetermined temperature while the second ply is a highly filled microporous sheet which is substantially stable at that temperature. U.S. Pat. No. 4,650,730 (the teachings of which are expressly incorporated herein by reference) forms its sheet product by methods which include formation of individual sheets, bonding them together and providing either multiple extraction steps or a combination of extraction and processing steps. These multi-step methods provide a disincentive to forming the sheet product as a cost-effective means to provide safety to lithium battery designs.
The present invention is directed to an effective and efficient method of forming a two-ply microporous sheet product capable of having at least one ply of the sheet product transform to a substantially non-porous ply at a predetermined elevated temperature while retaining the length and breadth dimensions of the sheet product.